Who played Screech Powers in 'Saved by the Bell'?

Who played A.C. Slater in 'Saved by the Bell'? In which TV series is A.C. Slater a main character? Who played Kelly Kapowski in 'Saved by the Bell'? In which TV series is Kelly Kapowski a main character? In which TV series is Screech Powers a main character? Who played Steve Urkel in 'Family Matters'? In which TV series is Steve Urkel a main character? Who played Sam Beckett in 'Quantum Leap'?